This application claims the priority of Japanese Patent Application No. 2003-100324, filed on Apr. 3, 2003, in the Japanese Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a light scanning unit. More particularly, the present invention relates to a light scanning unit suitable for an exposure device for an electrophotographic apparatus, such as a color printer having a plurality of exposed objects.
2. Description of the Related Art
An electrophotographic exposure devices can be categorized into devices that use a laser diode and devices that use a light emitting diode (LED). Devices that use an LED typically perform an exposure process by mapping one LED on 1 dot of a pixel of an image to be recorded in an exposed object. In general, devices using the LED use a light source called an LED head in which a plurality of LEDs are arranged.
The light source is configured in such a manner so as to arrange an LED chip in which a plurality of LED's are formed on a substrate and to form a plurality of LED arrays. Japanese Patent Publication No. Hei 10-035011 (published on Feb. 10, 1998) entitled “Light Emitting Diode Array and Fabrication thereof” discloses this type of configuration. The disclosed apparatus condenses light on an image-formed surface by arranging an optical system between an LED and an exposed surface of an exposed object.
In addition, devices using a laser diode scana laser beam in a main scanning direction on an exposed surface of an exposed object using a light scanning unit.
Devices using a laser diode use an F-θ lens to maintain the same scanning speed and the same beam shape on an exposed surface. Japanese Patent Publication No. Hei 09-096769 (published on Apr. 8, 1997) entitled “Method and Device for adjusting Optical Axis of Optical Scanner and Optical Scanner” discloses this type of configuration.
In the disclosed apparatus, a laser beam emitted from a laser diode is diffused light, and the laser beam is collimated by a collimator lens. The shape of the collimated laser beam is restricted in a slit, and the laser beam is condensed by a cylinder lens in a subscanning direction on a reflective surface of a polygonal rotating mirror which is a light scanning unit. Subsequently, light scanned in a main scanning direction on the polygonal rotating mirror is condensed by an F-θ lens (or lens group) on the exposed surface of an exposed object and is scanned at a uniform speed.
An electrophotographic exposure device using an LED can be made smaller. However, devices using an LED have known problems. First, a plurality of LED chips need to be precisely arranged on a substrate. Second, the circuit is complex. Third, due to the characteristics of an optical system, the distance from each LED chip to an exposed surface of an exposed object should be precise. Fourth, the degree of nonuniformity of light quantity between LED chips needs to be corrected.
In addition, in order to scan a laser diode light source using a polygonal rotating mirror onto an exposed surface, the spot of light projected onto an exposed surface is small, a circuit is relatively simple, and the depth of a focus is deep. As a result, the distance precision with respect to an exposed surface is comparatively low.
Meanwhile, recently, color electrophotographic printers has been developed rapidly. In particular, electrophotographic printers need to form an image four times, compared to a conventional black/white printer, so as to form an image having four colors, such as cyan, magenta, yellow, and black, into one color image.
Electrophotographic color printers can be categorized into two types. They are single pass electrophotographic color printers and multi-pass electrophotographic color printers. The single pass electrophotographic color printer mounts one exposure device in one drum, performs a development process using a four-color developer, superimposes an image on an intermediate transfer body, and transfers the superimposed image onto a sheet of paper.
The multi-pass electrophotographic color printer mounts four developers and four exposure devices, and four photosensitive bodies. Thus, the multi-pass electrophotographic printer is mechanically like four conventional black/white printers superimposed on one another.
In the single pass electrophotographic color printer, the output speed is reduced to ¼, so as to superimpose an image four times, such that the single pass electrophotographic color printer has a low speed. Also, the mechanism for moving a developer is complex. However, the single pass electrophotographic color printer can configure a photosensitive drum and an exposure device as a single body.
The printing speed of the multi-pass electrophotographic color printer is fast. However, the multi-pass electrophotographic color printer requires four exposure devices and four photosensitive bodies and thus, the structure becomes complex.
The present invention relates to an exposure device for a multi-pass electrophotographic color printer.
A conventional multi-pass electrophotographic color printer has the same number of laser diodes as the number of exposed objects and also requires the same number of polygonal rotating mirrors or F-θ lenses as the number of exposed objects. As a result, the size of an apparatus becomes large, and the costs of manufacture are increased.
In addition, in general, the refractive index of light beams is varied by the wavelength of light traversing a lens. If the wavelength of light is shortened, the refractive index of the light beam is increased. If light beams having different wavelengths are incident on the same lens, the light beam having a short wavelength is focused at a short distance, while the light beam having a long wavelength is focused at a longer distance. Thus, in order to allow light having a short wavelength and light having a long wavelength to have the same beam diameter, a focal distance of light having a long wavelength needs to be adjusted before and after the focal distance of light having a short wavelength.
In addition, since the main scanning magnification of a light beam having a short wavelength is reduced in the same position, a large correction of the main scanning magnification is needed. In order to realize large magnification correction, large correction, such as large-modulating of a data period or varying of a data width, is needed.